1. Technical Field of the Invention
The invention relates to an image forming device for registration of images built up from pixels (pixel patterns), which image forming device is provided with one or more image registration units each including a number of image forming elements and electronic drivers for separately energizing each image forming element to form the pixels. The invention also relates to an improved image registration unit for use in such an image forming device.
2. Description of Related Art
Image forming devices as described above are known in various technologically different embodiments. There are electrostatic or electrophotographic image forming devices wherein an electrostatic charge image is formed on a dielectric or photoconductive medium by means of one or more image registration units consisting of an array of separately energizable electrodes or an array of LEDs. The charge image is then developed with toner powder, and the powder image is transferred to a receiving support and fixed thereon.
Magnetographic image forming devices are comparable to the above electrostatic or electrophotographic image forming devices and by means of an array consisting of minuscule magnetic heads write a latent magnetic image on a magnetizable medium, this image is developed with magnetically attractive toner powder, and the toner powder image is finally in turn transferred and fixed on an image receiving material.
Another known image forming technology, in which images built up from pixels are recorded, is the ink jet technology. In this technology, image registration units are used which contain fine ink ducts or ink tubes, which again can each be driven separately to eject fine ink droplets on to an image registration medium. The inkjet technology is known in many embodiments which will not be discussed in greater detail here, except to say that in all these embodiments image registration units are used which, just like the image forming units used in the above image forming technologies, are equipped with electronic drivers by means of which each ink-ejecting ink tube (image forming element) can be energized separately to eject one or more ink droplets.
A typical example of an image forming unit including a number of image forming elements, in this case each consisting of an endless circular electrode, and electronic drivers for separately energizing each image forming element (circular electrode) is described in European Patent Application No. 0 595 388. The production of the image forming unit, and the device in which it is used, are described in detail in the above patent application, so that reference should be made to this publication for further information.
The invention will hereinafter be described primarily with reference to an image forming device and image forming unit of the type known from EP 0 595 388, but can also be used in other image forming units and devices, for example of the type as indicated hereinabove in the introduction. The image forming unit as described in EP 0 595 388 contains a large number of circular image forming elements which are substantially parallel to one another and which are electrically connected to electronic drivers disposed on a control element. The production of the image forming unit includes a number of steps of varying criticality, which are intended finally to result in an image forming unit of long life, i.e. the separate image forming elements must be permanently connected to their separate driver and no short-circuits are permissible between two or more adjacent image forming elements nor may they form quickly after the image forming unit has been put into use. Despite the great care devoted to the production process, it does happen that the image forming unit produced does have one or more weak spots not observed in the production process, for example one or a few small air bubbles enclosed in the insulating resin layers applied, and after a relatively small number of image loads these air bubbles can result in short-circuits between two image forming electrodes or a break in the electrical contact between the image forming element and the electronic driver.
The results of these faults are directly visible on the prints produced, so that the expensive image forming unit has to be replaced. There is a constant attempt to improve the production process further for the image forming unit, but in practice this will never give an ideal situation in which only permanent image forming units are obtained. There is accordingly a need for an image forming device in which quality faults in the image registration due to the breakdown or malfunctioning of one or more image forming elements can be dispensed with or can be made practically invisible on the print without directly replacing the image forming unit.
According to the invention, an image forming device has electronic drivers that are provided with a check circuit by means of which the output characteristic of the electronic driver can be measured and a control device which by reference to the measured output characteristic can determine proper operation or malfunctioning of the image forming element controlled by the driver. If the image forming element is found to operate incorrectly, the control device takes corrective action to eliminate the malfunction or so influence the image forming process that the visual effect of the inadequate operation of the image forming element on the resulting print is reduced.
By providing the drivers according to the invention with a check circuit by means of which the output circuit of the driver can be measured, the control device of the printing apparatus can monitor the operation of each image forming element. By comparing the measured value of the output voltage and/or shape of the output voltage pulse with a reference, it is possible for the control device to determine which image forming element or elements do not operate properly and what the nature of the malfunction is, and then on the basis thereof control a correction step so that the result of the fault on the finally obtained print is invisible or is barely perceptible. As a result, the consistently expensive image forming element does not have to be replaced directly in the event of breakdown of just one image forming element or even of a number of image forming elements distributed over the total width of the image forming unit.
The correction step which can be carried out in order to remedy the defects occurring and for rendering the same invisible or less visible on the print, will often depend on the type of image forming element. One step which is possible for all types of image forming elements (electrostatic, magnetic, LED, ink jet) is to provide the image forming device with one or more smaller auxiliary image forming units, which each comprise only a small number of separately energizable image forming elements and which are displaceable in the image forming plane so that when permanent breakdown of one or more image forming elements is found, e.g. due to a break in the electrical connection, they can be brought into operation instead of the faulty image forming element and take over the function there of the image forming element which is no longer operative.
In the case of a magnetic printing apparatus, an ink jet device or an electrostatic printing apparatus equipped with pin electrodes, it may occur that an image forming element will not operate properly or will operate poorly as a result of soiling or, in the case of an ink jet device, due to clogging or the presence of an air bubble in an ink jet tube. If the control device by measuring the output characteristic of the drivers detects a malfunction of this kind, a cleaning device provided or an air bubble eliminating process can be activated to eliminate the malfunction. The control unit can then again check whether the problem really has been solved, by re-measuring the output characteristic of the driver or drivers concerned and if necessary repeat the cleaning and measuring steps.
Apart from cleaning image forming elements or replacing them by bringing an auxiliary element into position, the result of a fault on an image forming element can also frequently be made practically invisible by appropriate image processing. This is the case for example in image forming units of the type described in European Patent Application 0 595 388. A malfunction occurring with these image forming elements is short-circuiting between two adjacent image forming elements. The results of the short-circuit can be solved via the image processing by the image forming unit regarding the short-circuited image by forming elements as a single image forming element and, at each image line for writing, determining, on the basis of details concerning adjacent image pixels, whether the xe2x80x9cimage forming elementxe2x80x9d should or should not be energized. The short-circuited image forming elements are then either energized or not energized simultaneously in each image line for registration. The result of this is a local resolution reduction, but since this resolution reduction is restricted to just a few pixels, it is only very rarely disturbingly perceptible on the prints.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.